Method of producing fine tungsten powder from tungsten oxides

ABSTRACT

The present invention relates to an improvement in the method and the apparatus for reducing tungsten oxide powder to tungsten metal powder. The method produces more uniform particle size distribution by decreasing the variability of temperature and inconsistency of reductant gas flow during the reduction process. The method includes the steps of: providing powder particles of WO x  in a unique reaction boat with a low aspect ratio to a multi-tube furnace; (a) contacting particles of WO x , wherein x is at least 2.5, with hydrogen and water vapor under fluid bed conditions at a temperature in the range of about 600 degrees to about 1000 degree Centigrade and at a water partial pressure equal to or greater than the equilibrium partial pressure, whereby said particles of WO x  are reduced essentially to particles of WO 2  with substantially no formation of tungsten metal, and (b) reducing the partial pressure of water in the fluid bed to a level sufficient to achieve reduction of said particles of WO 2  to tungsten metal while continuing to maintain said bed at a temperature which is substantially equal to or higher than said first temperature but not over about 1000 degree Centigrade.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 60/352,789, filed Nov. 20, 2001.

FIELD OF INVENTION

[0002] This invention relates to a process for the preparation of powdered tungsten metal, and more particularly a process employing a push through oven and reaction boats with low aspect ration for producing tungsten metal of controlled particle size from particles of tungsten oxide.

BACKGROUND OF THE DISCLOSURE

[0003] Tungsten and its alloys are widely used in high temperature structural elements, including lamp filaments, other electrical elements, and electrical instruments. Tungsten is also used as an alloying element with other metals in the manufacture of structural parts in which strength, ability to withstand high temperatures, and oxidation and corrosion resistance, particularly at high temperatures, are required. Fine particles of tungsten are used to make metallic composites from various metal powders using a sintering process.

[0004] Tungsten metal powder is the starting material of choice for all the above products. The tungsten metal powder should be highly pure and preferably of substantially uniform particle size. High purity is readily obtained by present methods, but uniformity of particle size is not always achieved. Wide particle size distribution complicates fabrication into many goods and products.

[0005] Another commercial process for preparing tungsten uses a rotary process for converting APT to tungsten metal in a single step. This process requires continuous and accurate control of APT feed rate. Product quality is also dependent upon APT feed rates. Throughput is related to the desired particle size. Throughput of a typical rotary furnace can vary from 20 kilograms per hour for fine particles to 70-80 kilograms per hour for coarse particles. In practice, the single step rotary reduction requires approximately the same degree of process control as the first process and is slightly less costly. On the other hand, particle size distribution is generally even broader than that in the first mentioned process.

[0006] Tungsten metal powder is produced commercially in a two step process in which ammonium paratungstate (APT) is converted to a tungsten oxide in particle form in a rotary tube furnace, and in which the oxide particles, contained in rectangular boats, are reduced to tungsten metal powder in a stationary tube furnace. The tungsten oxide particles obtained in the first step are chemically non-uniform and are of varying particle size, even with carefully controlled furnace conditions. Tungsten metal powder obtained in the second step is likewise not uniform in particle size. This is due to variations in the starting oxide, in temperature and relative hydrogen/water contents within the particle bed. It is not possible to obtain a uniform product with narrow particle size distribution according to this method. A uniform product with broad particle size distribution is achieved only by blending various lots of powder, since particle size distribution of the tungsten metal powder as produced varies from lot to lot.

[0007] Other processes have been reported in the literature but have not gained wide acceptance. For example, U.S. Pat. No. 3,324,007 describes a fluidized bed process for obtaining tungsten metal powder from tungsten hexafluoride. Early attempts to prepare tungsten metal powder by fluid bed techniques resulted in very broad (and hence less desirable) particle size distribution than those obtained in stationary and rotary furnaces. Additionally, U.S. Pat. No. 5,125,964 teaches a fluidized bed process for producing tungsten metal from particles of tungsten oxide wherein said tungsten oxide is reduced in a fluidized bed reaction zone in the presence of a mixture of hydrogen and water and wherein water is added to said reaction zone in an amount sufficient to maintain sufficient water in said zone to achieve the desired level of reduction of said tungsten oxide particles to tungsten metal.

[0008] What is needed in the art is a method of producing Tungsten metal powder of controlled particle sizes.

SUMMARY OF THE PRESENT INVENTION

[0009] The present invention relates to an improvement in the method and the apparatus for reducing tungsten oxide powder to tungsten metal powder. The method produces more uniform particle size distribution by decreasing the variability of temperature and inconsistency of reductant gas flow during the reduction process. The method includes the steps of: providing powder particles of WO_(x) in a unique reaction vessel with a low aspect ratio compared to a multi-tube furnace; (a) contacting particles of WO_(x), wherein x is at least 2.5, with hydrogen and water vapor under fluid bed conditions at a temperature in the range of about 600 degrees to about 1000 degree Centigrade and at a water partial pressure equal to or greater than the equilibrium partial pressure for the reaction

[0010] whereby said particles of WO_(x) are reduced essentially to particles of WO₂ with substantially no formation of tungsten metal, and (b) reducing the partial pressure of water in the fluid bed to a level sufficient to achieve reduction of said particles of WO₂ to tungsten metal while continuing to maintain said bed at a temperature which is substantially equal to or higher than said first temperature but not over about 1000 degree Centigrade.

[0011] In addition to the low aspect ratio mentioned above, the unique reaction boat of the present invention further comprises a support portion that keeps the reaction vessel from deforming during the reduction process, and cavity of the reaction vessel is shaped to allow a stream of reductant gas to flow over and through tungsten oxide that was therein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

[0013]FIG. 1 is a schematic illustration of a multi-tube furnace used in the prior art;

[0014]FIG. 2 is a schematic illustration of the low aspect ratio reaction boats;

[0015]FIG. 3 is a schematic representation of the furnace and reaction boats; and

[0016]FIG. 4 is a side view of the reaction furnace of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017]FIG. 1 shows a prior art method for producing tungsten metal where a multi-tube furnace 11 is used to heat and reduce tungsten oxide by flowing a reducing gas, such as H₂, over reaction boats that are generally partial tubes. This multi-tube furnace method for producing tungsten metal powder has problems controlling particle size due to the lack of uniformity of temperature. The tungsten oxide powder, if heated unevenly, will produce metal particles that vary from the powder particle size through a process of agglomeration. Furthermore, the depth of tungsten powder varies across the cross-section of each of the tubes, providing another reaction variables and decreases the uniformity of conversion of the tungsten oxide to tungsten metal.

[0018] Referring to FIG. 2, the unique pusher plate-tray 20 of the present invention is shown. The pusher plate-tray 20 comprises a reaction vessel 21 with an aspect ratio of at least 8 to 1, width to depth, are employed. Given this greatly increased width to depth ratio over previous practices, and the desire to use a push through mechanism in the reaction furnace, the improved pusher plate-trays 20 are made in two layers: a support portion 22 that keeps the reaction vessel 21 from deforming during the reduction process, and the reaction vessel 21 as mentioned above. In the preferred embodiment, the pusher plate tray 20 including the reaction vessel 21 and the support portion 22 is cast in one piece. The reaction vessel 21 further comprises a flat rectangular section 23 enclosed by four walls. The four walls further comprise an inner surfaces 24 a and 24 b, and an outer surface 25, wherein the outer surfaces 25 of all four walls are perpendicular to the flat rectangular surface 23 of the upper tray. The inner surfaces of two opposing walls 24 a are angled inward at an angle a to direct gasses toward the surface 23 of reaction vessel 21. The inner surfaces of the remaining two walls 24 b are substantially perpendicular to the flat rectangular surface 23. The support portion 22 further comprises a bottom surface 26. The bottom surface 26, or lower base serves as the contact point for the pusher plate tray and the chamber. The support portion 22 serves to provide a sturdy base for the tray, while minimizing the contact area between the pusher plate and the surface of the chamber. The support portion 22 has the same or nearly the same width and length dimensions of the reaction vessel 21 width and length and is designed to minimize the heat-sink losses to the tungsten oxide by providing a reinforced, but light weight support means that allows the reaction vessel portion to be built as lightly as possible. Both the support portion of the pusher plate 20 and the reaction vessel 21 can be made of temperature resistant alloys, ceramics or other materials capable of operating within the temperature range used for reduction of tungsten oxide to tungsten metal. Additionally, in the preferred embodiment the pusher plate 20 including the reaction vessel 21 and the support portion 22 is comprised of a single integral material. The support portion 22 may be a honeycomb configuration as shown in FIG. 2, or any other configuration that satisfies the thermal and support criteria mentioned above. Additionally, other embodiments are contemplated where inner walls 24 a are curved as opposed to the straight inner wall 24 a illustrated in FIG. 2. In the preferred embodiment the thickness of the thickness of the flat rectangular section 23 is 2 mm.

[0019]FIG. 3 illustrates the operation by which the pusher plate-tray 20 is feed into the furnace. The process begins by supplying the pusher plate 20 with a layer of tungsten oxide. The height of the layer of tungsten oxide should be between 1 and 12 mm. The ratio of the height of the tungsten oxide to the width of the pusher plate-tray 20 is between 1:10 and 1:600. With the opening the entrance door 32 a of the first chamber 32, pusher plate-tray 20 with tungsten oxide is carried on the transporting conveyer 31 via conveyer pusher 31 a into the first chamber 32. Once the tray 20 is inside of the chamber 32 the entrance door 32 a is closed. Upon closing of the first door 32 a, the atmosphere of the first chamber 32 is replaced by the furnace atmosphere. This is accomplished by opening exit door 32 b after closing of the first door 32 a. The Tray 20 is then pushed through the exit door 32 b by loading pusher 33. After the tray 20 passes through the exit door 32 b, the exit door 32 b is then closed. Finally, the pusher plate tray 20 is feed into the furnace 34 by the main pusher 35.

[0020] In a specific embodiment the reaction is carried out in at least two stages to produce tungsten metal powder of controlled particle sizes. Referring now to FIG. 4, a side view of the reaction furnace 34 of the present invention is shown. The main pusher 35 directs the pusher plate tray 20 from the chamber entrance 45 towards the chamber exit 47. The reaction furnace of a specific embodiment comprises multiple, cross-flow inducing, inlet and outlet ports for providing a flow of gas as required. The pusher plate tray 20 first enters Zone 1 to thereby commence the initial phase 41. While in the initial phase 41 the pusher plate tray 20 containing tungsten oxide is subjected to chamber temperatures ranging from 600° C. to 700° C. Additionally, H₂ gas passes over the tray 20 in a direction opposite the flow of the tray to further produce water. After the initial phase the tray enters a transitional period wherein the dewpoint of the tungsten oxide is lowered with the removal of the reductant gas. Additionally, during this transitional period, additional dry H₂ gas, electrically preheated to match the furnace temperature is introduced providing a cross flow of reductant gas across the reaction vessel. After the transitional period, the tray 20 enters Zone 2 or the high temperature phase 42. During this high temperature phase 42 the tungsten oxide is subjected to temperatures between 900° C. and 1100° C., as well as a counter-current flow dry H₂ gas. However, in addition to the counter-current passage of the H₂ gas, a cross-flow of H₂ gas across the reactant bed is also produced.

[0021] Furthermore, the rectangular reaction vessel has front and rear ends configured to cooperate with the ends of the other trays as they are being pushed through the reducing furnace. Thus the front of a rectangular reaction vessel may be configured to fit against, or within, the rear edge of the rectangular reaction vessel in front. 

What is claimed is:
 1. Apparatus for the production of tungsten metal powder using a push through furnace comprising: rectangular reaction vessels with two sidewall, a front wall, a rear wall, and a bottom forming a cavity, and with a length width and height, and the ratio of said width to height is at least 8; and said cavity is shaped to allow a stream of reductant gas to flow over and through tungsten oxide that was therein placed.
 2. The apparatus for the production of tungsten metal powder of claim 1, wherein said bottom of said reaction vessel is between 0.5 mm and 4 mm thick.
 3. The apparatus for the production of tungsten metal powder of claim 1, wherein said bottom of said reaction vessel is 2 mm thick.
 4. The apparatus for the production of tungsten metal powder of claim 1, further comprising a reaction vessel for holding the tungsten oxide, and a support portion, below said reaction vessel for contacting the furnace surface.
 5. The apparatus for the production of tungsten metal powder of claim 4 wherein said support section is formed in a honeycomb configuration.
 6. The apparatus for the production of tungsten metal powder of claim 4 wherein said apparatus is cast in one piece.
 7. The apparatus for the production of tungsten metal powder of claim 1 wherein said front and said rear walls further comprise an inner surface, wherein said inner surface is inclined at an angle to promote said stream of said reductant gas to flow over and through said tungsten oxide.
 8. The apparatus for the production of tungsten metal powder of claim 7 wherein said angle of said inner surface is a 45°.
 9. A method of producing tungsten metal powder comprising the steps of: providing powder particles of WO_(x) into a reaction boat with a low aspect ratio; contracting said powder particles of WO_(x) wherein x is at least 2.5, with hydrogen gas and water vapor under fluid bed conditions at a temperature in the range of 600 degrees to 1000 degree Centigrade; and reducing the partial pressure of water in the fluid bed to a level sufficient to achieve reduction of said particles of WO₂ to tungsten metal.
 10. The method of producing tungsten metal powder of claim 9, further comprising the step of continuing to maintain said bed at a temperature which is substantially equal to or higher than said first temperature but not over about 1100 degree Centigrade.
 11. The method of producing tungsten metal powder of claim 10, further comprising the step providing dry H₂ gas through multiple, cross-flow inducing, inlet and outlet ports of a muffle.
 12. A method of producing fine tungsten powder from tungsten oxide comprising the steps of: providing a muffle furnace with rectangular cross section trays; loading said trays with a thin layer of tungsten oxide; reducing said tungsten oxide under a hydrogen atmosphere; and producing said fine tungsten powder.
 13. The method of producing fine tungsten powder from tungsten oxide of claim 12 wherein said thin layer of tungsten oxide is between 1 mm and 12 mm high. 